1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a printer, copier and the like.
2. Description of the Related Art
Conventionally, as an image forming apparatus such as a copier or a printer using the electrophotographic system, there has been known an intermediate transfer type image forming apparatus using an intermediate transfer belt. The intermediate transfer type image forming apparatus can form color images (multiple image) on a sheet by means of a primary transfer process and a secondary transfer process. Additionally, there has been known another image forming apparatus which has a sheet conveying belt for conveying the sheet so as to transfer a toner image onto the sheet carried by the sheet conveying belt.
For example, Japanese Patent Application Laid-Open No. 2002-23512 has proposed an art for controlling the bias of a transfer means based on a result of detection of the impedance of the transfer means for such an image forming apparatus.
There is still another method in which the impedance of the entire transfer portion is detected by a constant current control before a recording material 211 reaches a transfer region and based on that detection result, a voltage applied to a transfer member for transferring a developing agent, when a recording material 211 reaches a transfer region, is controlled to a constant level. This constant voltage control method is called active transfer voltage control (ATVC) method hereinafter.
As the method for controlling a voltage applied to this transfer member, for example, Japanese Patent Application Laid-Open No. 02-123385 has proposed a control method in which a constant current is applied to the transfer portion in a predetermined time period, and a voltage calculated and determined based on a preliminarily set control equation and a voltage generated at that time is applied to the sheet if the transfer member exists.
However, when the transfer sheet is used as a transfer member as shown in FIG. 16, there exists the following inconvenience. FIG. 16 shows a view for explaining a load applied to a primary transfer portion of an image forming apparatus having the intermediate transfer body. FIG. 16 shows a portion around the primary transfer portion. Reference numeral 222 denotes a photosensitive drum, which carries a toner image, and reference numeral 228 denotes an intermediate transfer belt to which the toner image is to be transferred from the photosensitive drum 222. Reference numeral 227 denotes a primary transfer sheet which executes primary transfer of the toner image from the photosensitive drum 222 to the intermediate transfer belt 228. If an image formation operation of the image forming apparatus is executed for a long period, the primary transfer sheet 227 is worn by friction on the intermediate transfer belt 228. As a result, a resistance value R of the primary transfer sheet 227 is increased.
Because a current supplied to the transfer portion which is the primary transfer sheet 227 is controlled to a constant level under the above-described ATVC control, a voltage V applied to the primary transfer sheet is increased according to following Ohm's law as the resistance value R of the primary transfer sheet 227 is increased.V=R×I  (1-1)
Where V is the value of a voltage (primary transfer bias) to be applied for the primary transfer, R is a resistance value of the primary transfer sheet 227 and I is the value of a current supplied to a primary transfer sheet 227.
When the primary transfer bias V is increased as described above, a primary transfer electrostatic attraction force F between the intermediate transfer belt 228 and the primary transfer sheet 227, generated in the primary transfer portion is increased according to the following Coulomb's law.F=q×E  (1-2)
Where F is a primary transfer electrostatic attraction force acting in a direction in which the primary transfer sheet 227 adsorbs the intermediate transfer belt 228, and q is a charge amount defined as 1.6×10−19 [C (Coulomb)]. E means an electric field which is generated in a direction to the primary transfer sheet 227 from the photosensitive drum with an application of the primary transfer bias.
Thus, as the primary transfer electrostatic attraction force F is increased, a primary transfer portion load F′ generated in the primary transfer portion is increased according to the following equation.F′=μ×N  (1-3)
F′ means a primary transfer portion load acting in an opposite direction to a transfer direction of the intermediate transfer belt 228, μ means a dynamic friction coefficient generated between the primary transfer sheet 227 and the intermediate transfer belt 228 and N means a normal force acting in an opposite direction to the primary transfer electrostatic force. The normal force mentioned here is a force having the same magnitude as the primary electrostatic attraction force F.
FIG. 15 is a diagram for explaining changes of load torque of the intermediate transfer belt 228 with a durable drive of a color image forming apparatus. That is, the primary transfer portion load F′ described in FIG. 16 is increased with executing the image formation operation durably. As a result, a load torque of the intermediate transfer belt 228 is increased.
Generally, the following relation needs to be established between the drive torque Tm of a drive motor (not shown) which drives the intermediate transfer belt 228 and the load torque T of the intermediate transfer belt 228, so as to drive the intermediate transfer belt 228 by drive motor.Tm≧T  (1-4)
However, as a result of the durable execution of the image formation operation described above, the primary transfer portion load F′ is increased, so that the load torque T of the intermediate transfer belt 228 is also increased.
As a result, the following relation sometimes occurs.{tilde over (T)}m<T  (1-5)
FIG. 17 is a sectional view of the image forming apparatus when the image formation operation of the image forming apparatus having the primary transfer sheet is executed durably. Reference numeral 236b denotes a driven roller which is driven to convey an intermediate transfer belt 228 when the image formation operation is executed durably. Here, consider a case where the load torque T of the intermediate transfer belt 228 becomes larger than the drive torque Tm of a drive motor (not shown) by the durable image formation operation as indicated by the equation (1-5).
In this case, a loop is generated in the intermediate transfer belt 228 located between a drive roller 237 and the primary transfer portion disposed on the uppermost stream side of the intermediate transfer belt 228.
Because the entire peripheral length of the intermediate transfer belt 228 is not changed (not expanded or contracted), a driven roller 236 disposed in the downstream of the primary transfer portion in the lowest downstream is pulled to the drive roller side so that it is moved to a position indicated by reference numeral 236b shown in FIG. 17.
When the image formation operation of the image forming apparatus is executed durably as described above, a loop can be generated in the intermediate transfer belt 228. Consequently, a color deviation due to the loop of the intermediate transfer belt 228 can be generated or a defective image forming can be generated due to a contact of a loop of the intermediate transfer belt 228 with any member in the image forming apparatus. Additionally, sometimes, a scratch or damage occurs in the intermediate transfer belt 228 thereby reducing the service life of the image forming apparatus.
The degree of the above-mentioned loop does not depend on the conveying velocity of the intermediate transfer belt 228. The reason is that it has been known that the coefficient of dynamic friction μ does not depend on the conveying velocity largely as a natural law. As a countermeasure for generation of the loop in the intermediate transfer belt 228, use of a member composed of the material and configuration having a low dynamic friction coefficient μ as a member for the primary transfer can be mentioned.
However, such a countermeasure generates an extreme increase in cost of manufacturing any member relating to the primary transfer, for example, by several times to ten and several times.
The above-described problem occurs in an image forming apparatus using a sheet conveying belt also.